Abrasives composition, substrate and process for producing the same, and magnetic recording medium and process for producing the same

ABSTRACT

An abrasives composition comprises (i) one or more abrasives, (ii) one or more abrasion accelerators and (iii) water. The abrasive comprises intermediate alumina particles having a mean particle size of primary particles of 40 nm or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an abrasives composition. This inventionparticularly relates to an abrasives composition capable of producing asubstrate for a magnetic recording medium, which substrate has a verysmall value of surface roughness. This invention also relates to aprocess for producing a substrate, wherein the abrasives composition isused. This invention further relates to a glass-like carbon substratefor a magnetic recording medium, wherein the substrate has a very smallvalue of surface roughness, and a magnetic recording medium using theglass-like carbon substrate.

2. Description of the Related Art

Recently, most computers are provided with recording devices, such asmagnetic disk drives. With the increase in the amount of recordedinformation occurring in recent years, it is required that recordingdevices be capable of recording information at high densities. Themagnetic disk drive has a magnetic head and a magnetic disk. As a meansfor achieving the recording of information at high densities, it isnecessary to reduce the glide height of the magnetic head. For suchpurposes, it is necessary that the surface roughness (Ra) of themagnetic disk be as small as possible.

Recently, glass-like carbon substrates have attracted particularattention as substrates for magnetic disks. The glass-like carbonsubstrates have a hardness higher than aluminum substrates, which arecurrently popular as the substrates for magnetic disks. Therefore, thethickness of the substrates can be reduced when made of glass-likecarbon. Also, glass-like carbon has a small specific gravity and islight in weight, and therefore glass-like carbon substrates are light inweight. Accordingly, substrates are very suitable for use in computers,which should be kept small in size and light in weight.

However, when glass-like carbon substrates are used as substrates formagnetic disks, it is difficult for the surface roughness of theglass-like carbon substrates to be reduced, due to the high hardness ofthe glass-like carbon. Specifically, if γ-alumina particles, typicallyused, are used in polishing step carried out during the production ofthe substrates for magnetic disks, the surface roughness of thesubstrates cannot be reduced (e.g., to less than 4 Å). This is becausethe α-alumina particles are hard and the mean particle size of theirprimary particles is at least 0.1 μm. If very soft abrasive is used inlieu of α-alumina particles, sufficient abrasive effects cannot beobtained, and the surface roughness cannot be reduced.

Japanese Patent Application Laid-Open 7-240025 proposes a method forreducing the surface roughness of a magnetic disk substrate. The methodcomprises the step of corroding a substrate material with a chemicalcorrosive agent, which reacts with the substrate material and softens aportion of the substrate material, and the step of abrading and removingthis portion of the substrate material using colloidal particles. Alsoproposed is a magnetic disk substrate having a surface roughness of lessthan 4 Å. However, it is difficult to reduce the surface roughness of aglass-like carbon substrate to less than 4 Å with the method proposed inthis patent application.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an abrasivescomposition capable of imparting a low surface roughness to a substrate,particularly a glass-like carbon substrate, so that information can berecorded on the magnetic recording medium at high densities.

Another object of the present invention is to provide a substrate, inparticular a substrate for a magnetic recording medium, having a verylow surface roughness, and a process for producing the substrate.

The inventors carried out extensive research in order to achieve theseobjects and found that the surface of a substrate can be efficientlyprocessed to the super-polished state by an abrasives compositioncontaining specific alumina particles. The present invention is based onthese findings.

The present invention provides an abrasives composition, comprising:

(i) one or more abrasives,

(ii) one or more abrasion accelerators, and

(iii) water,

wherein the abrasives comprise intermediate alumina particles having amean particle size of primary particles of 40 nm or less.

The present invention also provides a process for producing a substrate,comprising:

abrading surfaces of said substrate with an abrasives composition,

wherein said abrasives composition comprises

(i) one or more abrasives,

(ii) one or more abrasion accelerators, and

(iii) water, and

said abrasives comprise an intermediate alumina particle having a meanparticle size of primary particles of 40 nm or less.

The present invention further provides a substrate, comprisingglass-like carbon, wherein said substrate has a surface roughness Ra ofless than 4 Å.

The present invention still further provides a magnetic recordingmedium, comprising:

(i) the above-mentioned glass-like carbon substrate,

(ii) a magnetic layer on said substrate, and

(iii) a protective layer on said magnetic layer.

With the abrasives composition in accordance with the present invention,wherein the abrasives comprise the intermediate alumina particles havinga mean particle size of primary particles of 40 nm or less, a lowsurface roughness can be imparted to the substrate, so that informationcan be recorded on the magnetic recording medium at high densities.

In particular, with the abrasives composition in accordance with thepresent invention, high-hardness substrates, e.g. glass-like carbonsubstrates, for which the super-polishing process ordinarily cannoteasily be carried out, can be processed such that the surface roughnessof the abraded substrates may be less than 4 Å, providing very smoothsurfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic front view showing a double-sided polishingmachine, which may be used in a polishing step for a substrate for amagnetic recording medium; and

FIG. 2 is a view taken along line X--X of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Firstly, the abrasives composition in accordance with the presentinvention will be described in detail.

As described above, the abrasives composition in accordance with thepresent invention contains, as essential components, water, an abrasiveand an abrasion accelerator. Also, the abrasives composition inaccordance with the present invention is characterized by using, as theabrasive, one or more intermediate alumina particles having a meanparticle size of primary particles of 40 nm or less. The features of theabrasives composition in accordance with the present invention will bedescribed hereinbelow.

The abrasive used is an intermediate alumina particle having a meanparticle size of primary particles of 40 nm or less. The term"intermediate alumina particle" as used herein means alumina particlesother than α-alumina particles. Specifically, examples of intermediatealumina particle include γ-alumina particles, θ-alumina particles,δ-alumina particles, η-alumina particles, and amorphous aluminaparticles. If a substrate, particularly a high-hardness substrate (e.g.,a glass-like carbon substrate) is abraded using α-alumina particles asthe abrasive, it will be difficult for the surface roughness of theabraded substrate to be reduced to a very small value (e.g., less than 4Å). (Ordinarily, in such cases, the surface roughness of the abradedsubstrate is reduced to 10 Å at the lowest.) This is because theα-alumina particles are hard and their mean particle size is 0.1 μm ormore. Alternatively, if the abrasive processing is carried out so thatthe surface roughness of the abraded substrate may become very small,the time required to carry out the abrasive processing will become verylong, and pits are liable to occur on the surface of the abradedsubstrate. In contrast, when the intermediate alumina particle is usedas the abrasive, the surface roughness of the abraded substrate can bereduced (e.g., less than 4 Å) without any defects on the surface of theabraded substrate. The above-enumerated types of the intermediatealumina particle may be used alone, or two or more may be used incombination. Among the above-enumerated types of intermediate aluminaparticles, the γ-alumina particles and the θ-alumina particles arepreferable, and the γ-alumina particles are particularly preferable.

The mean particle size of the primary particles of the intermediatealumina particle is 40 nm or less. If the mean particle size of theprimary particles of the intermediate alumina particle is larger than 40nm, it will become difficult for the surface roughness of the abradedsubstrate to be reduced (e.g., less than 4 Å) when the abrasiveprocessing is carried out on the substrate, particularly a substratehaving high hardness, such as a glass-like carbon substrate. Noparticular limitation is imposed upon the lower limit of the particlesize of the intermediate alumina particle. However, if the particle sizeof the intermediate alumina particle is very small, the abrasion speedwill become slow, and the production efficiency will become low.Therefore, the mean particle size of the primary particles of theintermediate alumina particle is preferably at least 10 nm. The meanparticle size of the primary particles of the intermediate aluminaparticle is more preferably 15 to 30 nm.

The intermediate alumina particle having a mean particle size of primaryparticles of 40 nm or less, e.g. the γ-alumina particles, can beproduced by the known technique of heating ammonium alum NH₄ Al(SO₄)₂ !up to 900° C. Also known to persons skilled in the art is that theintermediate alumina particle can also be easily produced with any oneof various other techniques. The intermediate alumina particle iscommercially available under the trade name "0.02CR" (primary particlesize: 0.02 μm, supplied by BAIKOWSKI JAPAN CO.).

The mean particle size of the primary particles of the intermediatealumina particle may be measured by adding a dispersing agent to 0.1 gof the intermediate alumina particle, dispersing the intermediatealumina particle by subjecting to ultrasonic waves, drying the resultingdispersion, observing the dried intermediate alumina particle with ascanning electron microscope (SEM), and carrying out image analysis.

The intermediate alumina particle is used in the so-called "slurrystate," in the abrasives composition in accordance with the presentinvention. The content of the intermediate alumina particle in theabrasives composition in accordance with the present invention may beselected in accordance with the viscosity of the abrasives composition,the quality required for the product, or the like. Generally, thecontent of the intermediate alumina particle in the abrasivescomposition in accordance with the present invention preferably is 0.05%to 30%, by weight, and more preferably 0.3% to 25%, by weight. When thecontent of the intermediate alumina particle in the abrasivescomposition in accordance with the present invention falls within theaforesaid range, a substrate having a low surface roughness (e.g., asurface roughness of less than 4Å) can be obtained with good productionefficiency.

No limitation is imposed upon the pH value of the abrasives compositionin accordance with the present invention. However, the pH value of theabrasives composition is preferably 5 or lower, more preferably 2.0 to4.5, and most preferably 2.5 to 4.0. When the pH value of the abrasivescomposition is 5 or lower, oxidation occurs on the surface of thesubstrate during the abrasive processing step, which aids the mechanicalabrasion of the surface, and the surface roughness of the substrate canfavorably be adjusted to be very low. By way of example, in order forthe pH value of the abrasives composition to be 5 or lower, a metal salthaving an oxidizing group, which will be described later, an inorganicacid such as sulfuric acid or nitric acid, an organic acid, or the like,may be added to the abrasives composition.

An abrasion accelerator is present in the abrasives composition inaccordance with the present invention. By way of example, as theabrasion accelerator, a metal salt containing an oxidizing group maypreferably be used. The metal salt containing an oxidizing group has theeffect of promoting the oxidation of the surface of the substrate duringthe abrasive processing step and promoting the progress of themechanical abrasion of the surface. The various abrasion acceleratorsmay be used alone, or two or more may be used in combination. Generally,the metal salt containing an oxidizing group may be a metal halide, ametal salt of an organic acid, or a metal salt of an oxygen containingacid.

As the oxidizing group, any oxidizing group having oxidizing effects maybe used. Examples of oxidizing groups include a nitrate group, a sulfategroup, a sulfite group, a persulfate group, a chloride group, aperchlorate group, a phosphate group, a phosphite group, a hypophosphitegroup, a pyrophosphate group, a carbonate group, a lactate group, and anoxalate group. Among the above-enumerated oxidizing groups, the nitrategroup and the sulfate group are particularly preferable.

Examples of metals constituting the metal salts containing the oxidizinggroups include aluminum, magnesium, nickel, and iron. Among theabove-enumerated metals, aluminum and magnesium are preferable.

The content of the abrasion accelerator in the abrasives composition inaccordance with the present invention is preferably 0.05% to 30% byweight, and more preferably 0.3% to 20% by weight. When the content ofthe abrasion accelerator in the abrasives composition is within theaforesaid range, appropriate oxidizing effects can be obtained.Therefore, the substrate can be processed efficiently to a desired lowsurface roughness (e.g., of less than 4Å), and the production efficiencycan be increased.

The abrasives composition in accordance with the present invention canbe prepared by, for example, adding the intermediate alumina particle,the abrasion accelerator, and the other components which will bedescribed later, into water under stirring.

The content of water in the abrasives composition in accordance with thepresent invention is preferably 40% to 99.9% by weight, and morepreferably 85% to 99.5% by weight. When the content of, water in theabrasives composition falls within the aforesaid range, the substratecan be processed efficiently to a desired low surface roughness (e.g.,of less than 4Å), and the production efficiency can be increased.

If necessary, components other than those described above may also beadded to the abrasives composition in accordance with the presentinvention. Examples of such other components include a dispersing agentfor the abrasive, and a dispersing agent for fine powders which occur asa result of abrasion. The content of these components in the abrasivescomposition is preferably 0.5% to 10% by weight.

The substrate to be abraded with the abrasives composition in accordancewith the present invention is preferably a substrate for a magneticrecording medium, for example, a carbon substrate, in particular aglass-like carbon substrate. However, the type of the substrate is notlimited thereto. For example, the abrasives composition in accordancewith the present invention is also applicable to the abrasive processingof other substrates, e.g. a glass substrate such as a reinforced glasssubstrate and a glass ceramic, and an aluminum substrate.

When the abrasives composition in accordance with the present inventionis used for the polishing process carried out on the substrate for amagnetic recording medium, an abraded substrate having a low surfaceroughness, which is necessary to satisfy the recent demand for recordinginformation on a magnetic recording medium at high densities, can beobtained.

A preferred process for producing the substrate for a magnetic recordingmedium, in which the abrasive processing step is carried out with theabrasives composition in accordance with the present invention, will bedescribed below with reference to FIGS. 1 and 2, by taking thesuper-polishing process of a glass-like carbon substrate as an example.FIG. 1 is a schematic front view showing a double-sided polishingmachine, which may be used in a polishing step in the production of asubstrate for a magnetic recording medium. FIG. 2 is a view taken alongline X--X of FIG. 1.

With reference to FIGS. 1 and 2, a double-sided polishing machine 2 isprovided with a base 3 and a lower lap 4, which is located on the base 3and rotates in the direction indicated by the arrow A. A polishing pad 5is fitted to the upper surface of the lower lap 4.

As illustrated in FIG. 2, a plurality of planetary gear-like carriers 8are located on the upper side of the lower lap 4. Each carrier 8 isengaged with a sun gear 6, which is located at the center of the lowerlap 4 and rotates in the direction indicated by the arrow B, and aninternal gear 7, which is located on the outer circumference of thelower lap 4 and rotates in the direction indicated by the arrow C. Thuseach carrier 8 rotates around the sun gear 6 and around its own axis.Each carrier 8 has a plurality of holes, and glass-like carbonsubstrates 1 serving as work pieces are respectively set in the holes.

Also, as illustrated in FIG. 1, an upper lap 9 is located above thelower lap 4 and the carriers 8. An polishing pad (not shown) is fittedto the lower surface of the upper lap 9. The upper lap 9 is rotatablymounted on a bracket 11, which is fitted to the end of the output rod ofan air cylinder 10. The upper lap 9 can be moved up and down by the aircylinder 10. When the upper lap 9 is moved down, it is engaged withgrooves of a rotor 12 on the base side. The rotor 12 rotates in thedirection indicated by the arrow D in FIG. 2, and the associated upperlap 9 rotates in the same direction.

The abrasives composition in accordance with the present invention issupplied from a slurry feed pipe (not shown) to the position between theupper lap 9 and the lower lap 4.

When the upper lap 9 is moved down by the air cylinder 10, theglass-like carbon substrates 1, which move together with each carrier 8,are sandwiched between the lower lap 4 and the upper lap 9. In thismanner, the abrasive processing is carried out.

In general, the super-polishing process of glass-like carbon substratesmay be carried out with the aforesaid double-sided polishing machineunder the conditions described below.

Specifically, the processing pressure is preferably 10 to 2,000 g/cm²,and more preferably 30 to 1,500 g/cm².

The processing time is preferably 2 to 120 minutes, and more preferably2 to 30 minutes.

The hardness JIS A(JIS K-6301)! of the abrasion pads fitted to theplatens of the double-sided polishing machine is preferably 40 to 100,and more preferably 60 to 100.

The rotation speed of the lower lap of the double-sided polishingmachine may vary in accordance with the size of the polishing machine.For example, as for a 9B type double-sided polishing machinemanufactured by SPEED FAM CO., the rotation speed of the lower lap ispreferably is 10 to 100 rpm, and more preferably 10 to 60 rpm.

The supplying rate of the abrasives composition may vary in accordancewith the size of the polishing machine. For example, as for the 9B typedouble-face polishing machine manufactured by SPEED FAM CO., thesupplying rate of the abrasives composition is preferably 5 to 300cc/min, and more preferably 10 to 150 cc/min.

When the super-polishing process is carried out on glass-like carbonsubstrates, i.e. the abrasive processing is carried out on the surfacesof a glass-like carbon substrate, under the conditions described above,a glass-like carbon substrates, which are ordinarily difficult toprocess to smooth surfaces, can be processed to a surface roughness ofless than 4 Å.

The preferred process for producing the substrate for a magneticrecording medium, in which the abrasive processing step is carried outwith the abrasives composition in accordance with the present invention,has been described above. However, the production process is not limitedto the embodiment described above and may be applied to, for example,substrates other than the glass-like carbon substrates. In addition, theabrasives composition of the present invention is advantageous for theproduction of substrates other than a substrate for a magnetic recordingmedium, for example, a silicon wafer and various lenses which require alow surface roughness.

In the present invention, the magnetic recording medium can be obtainedby using the glass-like carbon substrate which has been obtained in themanner described above and has a surface roughness of less than 4 Å, andforming at least a magnetic layer and a protective layer, in this order,on the substrate. The surface roughness of the magnetic recording mediumreflects the surface roughness of the glass-like carbon substrate (i.e.,less than 4 Å) and can be reduced to a very small value. Therefore, themagnetic recording medium obtained in accordance with the presentinvention is advantageous over the magnetic recording medium which isprovided with a glass-like carbon substrate (having a surface roughnessof approximately 10 Å) obtained by conventional abrasion techniques, inthat the glide height of the magnetic head can be reduced to a verysmall value, and in that information can be recorded at high densities.

In the present invention, the surface roughness (center line meanroughness Ra) may be measured with an atomic force microscope.

In the magnetic recording medium of the present invention, the magneticlayer may be formed directly on the glass-like carbon substrate.However, it is preferred that at least one under layer is formed betweenthe glass-like carbon substrate and the magnetic layer, as described inExamples 4 to 6. The preferable material of the under layer is anonmagnetic, highly crystalline metal. Examples of preferable materialsfor the under layer include Ti, Cr, W, Si, Al, and alloys of thesemetals.

Also, in the magnetic recording medium, if necessary, a layer having anuneven surface may be formed between the glass-like carbon substrate andthe magnetic layer. The degree of unevenness of the layer is preferablyas low as possible from the viewpoint of the reduction in the glideheight of the magnetic head, provided that the magnetic head does notstick to the surface of the magnetic recording medium. For example, asfor a magnetic disk drive in which the contact start and stop (CSS)system is employed, the layer having an uneven surface is preferablylocated only in the CSS region (i.e., head landing zone). As for amagnetic disk drive in which the magnetic head does not come intocontact with the magnetic recording medium, it is preferable that thelayer having an uneven surface is not formed. The layer having an unevensurface may be formed with a technique for carrying out tape texturingonly at a prescribed area on the surface of the glass-like carbonsubstrate which has been obtained from the abrasive processing stepusing the aforesaid abrasives composition, a technique for texturingwith the spraying of fine particles, a technique for texturing withirradiation of a laser beam, or a technique for sputtering Al, Si, or anAl-M alloy, where M represents a metal having the capability to form acarbide.

As the material of the magnetic layer of the magnetic recording medium,Co-alloys containing Co as a principal constituent are preferable. TheCo-alloys comprising Cr (CoCr alloys) are more preferable, and theCo-alloys comprising Cr and Pt (CoCrPt alloys) are particularlypreferable. Specifically, CoCrPtB, CoCrPtTa, or the like, arepreferable.

As the material of the protective layer of the magnetic recordingmedium, carbons, particularly amorphous carbon and diamond-like carbon,are preferable.

In the magnetic recording medium, if necessary, a lubricant layer may beprovided on the protective layer. As the lubricant, for example, aperfluoroalkyl ether lubricant is preferable.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

EXAMPLES 1, 2, 3 AND COMPARATIVE EXAMPLES 1, 2, 3

An abrasives composition, as a slurry, was prepared by mixing 1% byweight of the abrasive listed in Table 1, 1% by weight of aluminumnitrate, serving as an abrasion accelerator (in Comparative Example 2,aluminum nitrate was not added), and the balance of water, and stirringthe resulting mixture. The abrasives composition obtained had a pH valuelisted in Table 1.

Thereafter, a glass-like carbon substrate, which had a diameter of 2.5inches and the surface roughness of approximately 12 Å with a polishingprocess, was set in the double-sided polishing machine. The surface ofthe glass-like carbon substrate was thus subjected to a super-polishingprocess with the prepared abrasives composition. The abrasive processingwith the double-sided polishing machine was carried out under theconditions shown below.

Setting conditions of double-sided polishing machine Double-sidedpolishing machine used: 9B type double-sided polishing machinemanufactured by SPEED FAM CO.

Processing pressure: 150 g/cm²

Processing time: 30 min

Hardness of polishing pad: 90

Rotation speed of lower lap: 40 rpm

Supplying rate of abrasives composition: 50 cc/min

The surface roughness Ra and the number of defects on the surface ofglass-like carbon substrate, obtained from the super-polishing process,were measured with the methods described below. The results are shown inTable 1.

Surface roughness Ra

The surface roughness Ra of 10×10 μm surface area was measured by usingan atomic force microscope (AFM) image (NANOSCOPE III manufactured byDIGITAL INSTRUMENTAL CO.)

Number of defects

The surface of the substrate was observed by using an optical microscope(×200), and the number of defects, having a size of 0.5 μm or larger onthe entire surface of the substrate, was counted.

EXAMPLES 4, 5, 6 AND COMPARATIVE EXAMPLES 4, 5, 6

With a sputtering process, a Ti layer (first under layer), a Cr layer(second under layer), a Co₇₆ Cr₁₂ Pt₈ B₄ layer (magnetic layer), and anamorphous carbon layer (protective layer) were successively formed topredetermined thicknesses on each of the glass-like carbon substratesobtained in Examples 1, 2, 3 and Comparative Examples 1, 2, 3. Further,a lubricant layer was formed to a predetermined thickness on theprotective layer by the dip-coating of FOMBLIN Z-03 manufactured byAUGIMONT CO. In this manner, a magnetic disk was obtained. As for theobtained magnetic disk, a glide height test (GHT) was carried out underthe conditions shown below. The results are shown in Table 2.

GHT

Magnetic Head: MG150T, 50% slider head manufactured by PROQUIP CO.

Passage rate: at a glide height of 1.0μ inch

Evaluation scale:

Passage rate of 95% to 100%; Very good

Passage rate of 70% to less than 95%; Good

Passage rate of less than 70%; Poor

                                      TABLE 1    __________________________________________________________________________                 Particle Size* Aluminum                                     Surface                                          Number of            Type of                 of Alumina                        pH of Abrasives                                Nitrate                                     Roughness                                          Defects            Alumina                 (nm)   Composition                                (wt %)                                     (Å)                                          (pieces)    __________________________________________________________________________    Examples          1 γ-Al.sub.2 O.sub.3                 20     3.8     1    3.2  0          2 γ-Al.sub.2 O.sub.3                 40     3.8     1    3.9  0          3 θ-Al.sub.2 O.sub.3                 30     3.8     1    3.9  0    Comparative          1 α-Al.sub.2 O.sub.3                 1000   3.8     1    12.1 0    Examples          2 γ-Al.sub.2 O.sub.3                 40     7.2     0    15.0 >500          3 γ-Al.sub.2 O.sub.3                 45     3.8     1    5.3  120    __________________________________________________________________________     *Mean particle size of primary particles

                  TABLE 2    ______________________________________                  GHT    ______________________________________    Examples      4     Very Good                  5     Very Good                  6     Very Good    Comparative   4     Poor    Examples      5     Poor                  6     Good    ______________________________________

As is clear from the results shown in Table 1, when the super-polishingprocess was carried out on the glass-like carbon substrates using theabrasive compositions in accordance with the present invention (Examples1, 2 and 3), no defect occurred on the surface of the abradedsubstrates, and the surface roughness of the abraded substrates could bereduced to a very small value (less than 4 Å). On the other hand, whenα-alumina particles were used as the abrasive (Comparative Example 1),though no defect occurred, the low surface roughness obtainable with theabrasive composition in accordance with the present invention could notbe obtained. Also, when the abrasive processing was carried out with anabrasives composition containing no abrasion accelerator (ComparativeExample 2), many defects occurred on the surface of the abradedsubstrate, and the low surface roughness obtainable with the abrasivecompositions in accordance with the present invention could not beobtained. Further, in cases where the abrasive processing was carriedout with the abrasives composition containing the abrasive having a meanparticle size larger than 40 nm (Comparative Example 3), defectsoccurred on the surface of the abraded substrate, and the low surfaceroughness obtainable with the abrasive compositions in accordance withthe present invention could not be obtained.

Also, as is clear from the results shown in Table 2, the magnetic disks(Examples 4, 5 and 6) which use the glass-like carbon substrates(obtained in Examples 1, 2 and 3) subjected to the super-polishingprocess using the abrasive compositions in accordance with the presentinvention, had a low substrate surface roughness and exhibited good GHTresults. On the other hand, the magnetic disks (Comparative Examples 4,5 and 6) which use the substrates obtained in Comparative Examples 1, 2and 3, exhibited GHT results inferior to those obtained with themagnetic disks of Examples 4, 5 and 6, because the magnetic disks inComparative Examples 4, 5 and 6 do not use substrates having a surfaceroughness as low as obtained in Examples 1, 2 and 3.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teaching. It is thereforeto be under stood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedtherein.

The priority documents of the present application, Japanese PatentApplications No. 7-292567 and No. 8-71577, filed on Nov. 10, 1995 andMar. 27, 1996, respectively, are hereby incorporated by reference.

What is claimed as new and is desired to be secured by Letters Patent ofthe United States is:
 1. An abrasives composition, comprising:(i) one ormore abrasives, (ii) one or more abrasion accelerators, and (iii)water,wherein said abrasives consist essentially of intermediate aluminaparticles having a mean particle size of primary particles of 40 nm orless wherein said intermediate alumina particles comprise an abrasiveeffective amount of at least one member selected from the groupconsisting of γ-alumina and θ-alumina.
 2. The abrasives composition ofclaim 1, wherein said abrasives composition has a pH of 5 or less. 3.The abrasives composition of claim 1, wherein said abrasion acceleratorcomprises at least one metal salt containing an oxidizing group.
 4. Theabrasives composition of claim 1, wherein said intermediate aluminaparticles have a means particle size of primary particles of 10 to 40nm.
 5. The abrasives composition of claim 1, comprising:40 to 99.9% byweight of said water, 0.05 to 30% by weight of said abrasionaccelerator, and
 0. 05 to 30% by weight of said intermediate aluminaparticles.
 6. The composition according to claim 1, wherein saidintermediate alumina particles comprise γ-alumina.
 7. An abrasivescomposition, prepared by mixing 40 to 99.9% by weight of water,0.05 to30% by weight of at least one abrasion accelerator, and 0.05 to 30% byweight of, one or more abrasives, wherein said abrasives consistessentially of intermediate alumina particles having a mean particlesize of primary particles of 40 nm or less and comprise an abrasiveeffective amount of at least one member selected from the groupconsisting of γ-alumina and θ-alumina.
 8. The abrasives composition ofclaim 7, wherein said intermediate alumina particle comprises γ-alumina.9. A process for producing a substrate, comprising:abrading a surface ofsaid substrate with an abrasives composition, wherein said abrasivescomposition comprises(i) one or more abrasives, (ii) one or moreabrasion accelerators, and (iii) water, and said abrasives consistessentially of intermediate alumina particles having a mean particlesize of primary particles of 40 nm or less and comprise an abrasiveeffective amount of at least one member selected from the groupconsisting of γ-alumina and θ-alumina.
 10. The process of claim 9,wherein said abrasives composition has a pH of at most
 5. 11. Theprocess of claim 9, wherein said abrading is carried out until saidsurface of said substrate has a surface roughness Ra of less than 4Å.12. The process of claim 9, wherein said substrate comprises amorphouscarbon.
 13. The process as claimed in claim 9, wherein said intermediatealumina particles comprise γ-alumina.